1. Field of the Invention
The present invention relates to image development devices in electrographic apparatus and more specifically to improved magnetic brush development configurations for such apparatus.
2. Description of the Prior Art
In electrographic apparatus of the kind in which an electrostatic latent image is formed on an image member (e.g. by exposure of a charged photoconductor member or stylus recording on a dielectric member), magnetic brush development is an extremely popular mode of developing the latent image (i.e. applying toner to render the latent image visible). Accordingly, a very large number of structural approaches have been devised to effect magnetic brush development.
Perhaps the most common general approach for magnetic brush development is to provide a non-magnetic cylinder which rotates to transport a developer mixture, comprising magnetic carrier particles bearing triboelectrically-attracted toner particles, from a supply to a development zone adjacent the image member. Stationary permanent magnets inside the cylinder attract the carrier particles to the outer surface cylinder so the carrier and toner will be transported by the rotating cylinder. In another similar mode, the outer cylinder is stationary and the interior magnets rotate to "walk" the developer along the cylinder surface from the supply to the development zone. Other variations provide movement of both the outer cylinder and the magnets or in some instances eliminate the outer cylinder and simply rotate a cylindrical magnet array.
The external magnetic fields, extending from poles of the magnet element of the magnetic brush, tend to form chains of carrier particles, which extend like many small fingers pointing outwardly from the brush. When these fingers move across the electrostatic-image-bearing surface of the member to be developed, toner is attracted from the carrier particles to the image member.
In general, the above-described magnetic brush techniques perform admirably; however, several problems have been encountered. For example, the fingers of developer which form the "nap" of the magnetic brush vary in length and thickness, e.g., because of non-uniformity in the magnet's composition (and thus field strength) or non-uniformity in the carrier particles' size or composition. This nap non-uniformity, in turn, causes uneven development of the contacted electrostatic image. Additionally, the upstanding carrier-chain fingers often scratch the developed image and the image member.
Because of this problem various approaches have been devised to "soften" the nap of magnetic brushes. For example, U.S. Pat. No. 3,457,900 discloses a magnetic brush configuration intended to cause a developer rollback, and consequent tumbling of developer, in the development zone. Another approach described in Research Disclosure No. 18319, pp. 357-358, January 1979 published by Industrial Opportunities, Ltd., Homewell, Havant, Hampshire, P09 1EF, United Kingdom, provides an A.C. magnetic field which agitates the nap of the magnetic brush at the development zone. Various prior art devices suggest other means to agitate such fingers of magnetic brush naps. These approaches are successful insofar as they have enhanced the uniformity of image development; however, these solutions give rise to at least one other problem. Specifically, these techniques can increase the likelihood and degree of developer "throw-off" from the magnetic brush. That is, as the developer chains are agitated, the centrifugal force acting on the developer due to the rotational velocity of the magnetic brush can overcome the magnetic attraction, and the carrier is thrown into the area around the development zone. Containment of these dustlike particles is difficult, and their abrasive nature present problems for long-life electrographic apparatus. The problems are even more severe where multicolor development is performed because inter-development-station contamination can occur.